Theoretical and experimental analysis of variable pressure filtration and the effect of side wall friction in compression-permeability cells

Theoretical and experimental studies were made in order to improve insight into the process of cake filtration. The mechanism of variable pressure filtration was analyzed by taking into account the variation of liquid and solid flow rates within the filter cake. Generalized expressions for internal flow rate variations, porosity, and pressure distributions throughout a filter cake were derived. A newly defined correction factor for Ruth's average filtration resistance was developed. A new numerical method for calculating the variation of variables involved in variable pressure filtration is presented. The phenomenon of side wall friction in compression-permeability cells was investigated. Transmitted forces, stress distributions, and the compression and permeability of the filter cake were measured experimentally and then analyzed. It was demonstrated that in applying the C-P cell test data to filtration neglect of the side wall friction effect could result in significant error. Observed stress distributions in the cakes differed from assumptions found in the literature. Doubt was cast upon a formula developed on the assumption of that vertical pressure is independent of cell radius. The methodology of C-P cell tests was found to be an important factor in conducting experiments. The present study shows that there are serious limitations in using the C-P cell as a device to obtain local porosities and local specific filtration resistances.